The name Agnes, as the wife of William "The Elder" McMurray is based upon the following analysis, listed in this Rootsweb.com post:

(Rationale for) Agnes as the wife of William McMurray: In a court case on 12 Mar 1756, "Jas. Simpson vs. James Campbell," I believe I have found a record that shows that Agnes was William's wife's name. The document filed refers to a Margaret Campbell "of the lower Cowpasture" (where the McMurray brothers' farms were located), a harpy that the citizens were mad at and who filed a petition in support of the plaintiff, John Simpson. The signers of the document are all neighbors of Samuel McMurray and Agnes & William McMurray (Chalkley, I, p. 306 & 315). I won't list all of the signers but other records confirm that they were the McMurrays' neighbors. The most important signers of this petition are, I think, Samuel McMory (McMurray), Agnes Memory (McMurray) and William Memory (McMurray). Note that by changing the "e" to "c" in Memory, it becomes McMory. This petition is the record, although circumstantial, that I use to support the name of Agnes as the mother of William McMurray's children. Not to mention the fact that Agnes is the name of one of Agnes's and William's daughters and that there are no other records for a family named "Memory" on the lower Cowpasture River.

Some claim his wife was named "Mary", based upon this early research error:

Mary as the wife of William McMurray: W. Fletcher Guy McMurry, a professor and an early and thorough researcher of the McMurray/McMurry family inadvertently started the misunderstanding about Mary. He mentioned in a letter to a cousin that he believed William's wife was named Mary because he found the name Mary in a deed. His comment has permeated the research of others that followed him and is included in Bone's McMurry Family. However, I have a subsequent letter that Fletcher wrote in which he made a note in the margin indicating the name Mary referred to a river in the deed to which he referred. William's land in the deed was on the River Mary, since renamed the South River on the floor of the Shenandoah Valley of Virginia.